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The Nov. 11, 2008, cable said Japanese officials were concerned about the reliability of the U.S. nuclear “umbrella” over Japan, “given the deteriorating nuclear situation around Japan, as North Korea continues to develop capabilities and China expands its arsenal.”

The umbrella concept is called “extended deterrence” and commits U.S. nuclear forces to defending Japan from nuclear strikes or threats.

“There are some in Japan that are discussing indigenous nuclear development in Japan, partly due to a lack of confidence in the U.S. extended deterrence,” said the cable from the U.S. Embassy in Tokyo, labeled “secret.”

The Japanese were assured by defense officials that the U.S. nuclear-deterrence policy “remains strong,” said the cable, made public on the anti-secrecy website WikiLeaks.

The cable also stated that Japan was stepping up military space operations. The proposed measures included bolstering space-based intelligence functions and increasing technology cooperation with civilian space companies.

The cable also stated that Japan was considering deployment of space “assets and technology, such as early-warning satellites, signal-intelligence satellites, compact reconnaissance satellites, and sensor and jamming resistant technologies.”